1. Field of the Invention
This invention relates to a tray for holding a product during heating in a dielectric oven, such as for cooing foodstuffs. Particularly, it relates to a tray for holding the product in such an oven, so that electrodes for producing an electromagnetic field are more easily maintained and inspected and energy is quickly and efficiently transferred to the product. Further, it relates to such trays for use in dielectric ovens having multiple support levels therein for heating commercial quantities of the product.
2. Description of the Related Art
Commercial ovens are colony convection ovens utilizing a slow convection heating process. Dielectric ovens, however, heat a product due to the electric, i.e., dielectric, losses caused when the product is placed in a varying electromagnetic field. If the product is homogeneous and the electromagnetic field is uniform, heat may develop uniformly and simultaneously throughout the mass of the product.
Ovens utilizing dielectric high frequency heating are known, and examples of such ovens are disclosed in U.S. Pat. Nos. 4,812,609 to Butot; 4,978,826 to DeRuiter et al.; and 4,980,530 to Butot, which are incorporated herein by reference. Such ovens may operate in a frequency range of 2 to 40 MHz. Referring to FIG. 1a, a dielectric oven 200 may be fitted with guide racks 202 for stacking a plurality of trays 204 carrying a product 206 to be heated. These racks 202 also may function as electrodes for producing the electromagnetic field.
Dielectric ovens may utilize an oscillating circuit or circuits requiring specially designed electromagnetic energy sources, e.g., power tubes. Such electromagnetic energy sources may be coupled and supply current to guide racks/electrodes 202 via contacts 205 which project into a heating cavity 208 through an oven housing 209. Such oscillating circuits generally provide a substantially fixed distribution of voltage and power within a heating cavity. Thus, longer heating times may be required for heating larger volumes of products. Further, frequencies at which the ovens may vary dependent upon the characteristics of the product being heated.
Referring to FIG. 1b, although dielectric ovens 200' may handle a plurality of vertically stacked trays 204', which permit products to be heated at multiple levels 210' within a single heating cavity 208', only a single pair of electrodes 202' may be provided to transfer the electromagnetic energy for heating. Thus, when a number of different heating levels 210' are used, the quantity of energy delivered to the product 206' in each tray may be reduced, and longer heating times may be required. As discussed above, electromagnetic energy sources may be coupled and supply current to electrodes 202' via contacts 205' which project into a heating cavity 208' through an oven housing 209'.
A dielectric oven may include a heating cavity defined by an oven housing for receiving a tray containing the product, a high frequency oscillating circuit having an electromagnetic energy source, e.g., a power tube, for generating a high frequency electric signal, and electrodes which may be coupled to the energy source and configured to produce an electromagnetic field in the cavity to transfer power from the oscillating circuit to the product. Such ovens may be operated to increase the power transferred from the oscillating circuit to the product, without increasing the operating voltage of the power source or the frequency of its operation. Moreover, such ovens may include a plurality of oscillating circuits having substantially similar resonant frequencies.
As noted above, the oscillating circuits may receive power from a power tube in order to establish respective oscillating signals. More particularly, at least first and second oscillating circuits may be provided, and the electrode configuration may include pairs of at least first and second electrodes which are respectively connected to the two oscillating circuits. The tray containing the product may be bracketed between electrodes of a capacitor in the oscillating circuit. The oscillating circuit may be arranged to provide a voltage across the capacitor which is twice the voltage across the power tube, thereby, permitting the doubling of distance between the electrodes of the capacitor without reducing the electromagnetic field strength. Thus, the quantities of the product which may be heated between the capacitor electrodes may be increased.
Each of the oscillating circuits may also include an inductance and a capacitance. The capacitance includes a pair of capacitors respectively formed between two electrodes of the oscillating circuit and a pair of capacitor plates or, for example, wall portions of the oven housing. Preferably, the two electrodes of each oscillating circuit are oriented to radiate an open electromagnetic field therebetween. In this configuration, a pair of interconnecting load capacitors is formed between the electrodes and capacitor plates of the oscillating circuits. The dielectric of the load capacitors includes the product placed between the electrodes. This configuration produces an open electromagnetic field between the electrodes of each of the pair of interconnecting (load) capacitors. The open electromagnetic field has a power intensity distribution determined by the dielectric characteristic of the product, while permitting the energy source to operate at a substantially constant power level. Further, the use of the load capacitors isolates the frequency of oscillation of the oscillating circuits from the effects of the dielectric characteristics of the product.
Electrodes affixed within a dielectric oven may be difficult to clean, maintain, inspect, and repair, and the oven may not be operated while electrodes are being removed for servicing or replacement. Further, if the electrodes function as racks, the electrodes are more susceptible to fouling, denting, and other damage. Fouling may be caused by the deposit of tray material on the electrodes as trays are slid onto and off of the electrodes. Fouling also may result from product residue or films of cleaning chemicals which may adhere to the tray and be deposited on the electrodes.
Uneven tray surfaces and improper or careless tray placement may cause dents and scoring of the electrodes. Such damage may adversely effect the uniformity of the electromagnetic field produced between the electrodes. Moreover, dents and scoring of the electrodes may increase the likelihood and severity of fouling because the dents and scoring may scrape residue or tray material from the tray sides and make cleaning the electrodes more difficult. Damage to the electrodes caused by denting and scoring may be exacerbated by subsequent fouling because the fouling deposits are within the electromagnetic field produced in the oven. Further, fouling also may result in localized pitting or general corrosion of the electrodes.
If the electrodes are separated from the tray, as depicted in FIG. 1b, and not used as tray racks, the oven may be made larger or the tray smaller. A larger oven would require greater energy to produce the same electromagnetic field strength. Smaller trays, however, would reduce the quantity of product heated and make the dielectric oven less economical.